Re: Online verification of checksums

On 3/4/19 4:09 AM, Michael Paquier wrote:
> On Sun, Mar 03, 2019 at 07:58:26AM +0100, Fabien COELHO wrote:
>> I agree that having a server function (extension?) to do a full checksum
>> verification, possibly bandwidth-controlled, would be a good thing. However
>> it would have side effects, such as interfering deeply with the server page
>> cache, which may or may not be desirable.
> 
> In what is that different from VACUUM or a sequential scan?  It is
> possible to use buffer ring replacement strategies in such cases using
> the normal clock-sweep algorithm, so that scanning a range of pages
> does not really impact Postgres shared buffer cache.
> --

But Fabien was talking about page cache, not shared buffers. And we
can't use custom ring buffer there. OTOH I don't see why accessing the
file through SQL function would behave any differently than direct
access (i.e. what the tool does now).

regards

-- 
Tomas Vondra                  http://www.2ndQuadrant.com
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services